There have been conventionally proposed various types of display devices, optical systems and the like that can display an aerial image. For example, an optical system described in Japanese Patent Laying-Open No. 2008-158114 (PTL 1) has a reflective plane-symmetric imaging element that causes light from an object to be projected to form an image at a plane-symmetric position.
The reflective plane-symmetric imaging element is formed into a plate and the reflective plane-symmetric imaging element is provided with a plurality of holes. The hole is formed to have a rectangular shape when viewed from above the hole. A mirror surface is formed on each inner surface of the hole and the adjacent mirror surfaces are arranged to vertically intersect with each other.
The light from the object to be projected enters the hole from one opening of the hole. The light that has entered the hole is repeatedly reflected in the hole and is emitted outside from the other opening of the hole.
When the reflected light that is emitted from the other opening and the incident light that enters the hole are observed from above the reflective plane-symmetric imaging element, the reflected light travels in the direction exactly opposite to that of the incident light. When the incident light and the reflected light are observed from a side of the reflective plane-symmetric imaging element, an incidence angle of the incident light is equal to a reflection angle of the reflected light.
Therefore, the light from the object to be projected forms the image at the plane-symmetric position with respect to the reflective plane-symmetric imaging element serving as a reference plane.
An optical element described in International Publication No. WO2007/116639 (PTL 2) is formed into a plate and has a configuration similar to that of the reflective plane-symmetric imaging element described in aforementioned Japanese Patent Laying-Open No. 2008-158114 (PTL 1). Therefore, this optical element causes light that has been emitted from an object to be projected and entered the optical element from one side of an element surface to form an image at a plane-symmetric position with respect to the optical element serving as a reference plane. Consequently, a real image is displayed in a physically insubstantial space on the other side of the element surface.
Furthermore, International Publication No. WO2007/116639 (PTL 2) describes a three-dimensional aerial image display device including this optical element. This three-dimensional aerial image display device includes a substrate functioning as a reflective image forming element and a display portion arranged on the lower side of the substrate. Light that has been emitted from the display portion and entered the substrate from a lower surface of the substrate forms an image above an upper surface of the substrate.
A three-dimensional aerial image display device described in Japanese Patent Laying-Open No. 2009-229905 (PTL 3) includes a two-plane corner reflector array and a display having a display surface that displays an image. The two-plane corner reflector array forms a real image of an object to be projected at a plane-symmetric position with respect to a geometric plane serving as a symmetric plane. This three-dimensional aerial image display device includes a driving unit moving the two-plane corner reflector array in the vertical direction.
A volume scanning three-dimensional aerial image display described in Japanese Patent Laying-Open No. 2009-75483 (PTL 4) includes a two-plane corner reflector array, a display having a display surface that displays an image, and a driving unit moving the display in the direction vertical to the display surface.